Page 205 - Machinery Component Maintenance

Machinery Alignment 187

Figure 5-12. Sag compensator,

Bracket Sag Effect on Face Measurements
Bracket sag is generally thought to primarily affect rim readings, with
little effect on face readings. Often this is true, but some risk may be
incurred by assuming this without a test. Unlike rim sag, face sag effect
depends not only on jig or bracket stiffness, but on its geometry.
Determining face sag effect is fairly easy. First get rim sag for span to
be used (we are referring here to the full indicator deflection due to sag
when the setup is rotated from top to bottom). This may be obtained by
trial, with rim indicator only, or from a graph of sags compiled for the
bracket to be used. Then install a setup with rim indicator only, on Cali-
bration pipe or on actual field machine, and “lay on” the face indicator
and accessories, noting additional rim indicator deflection when this is
done. Double this additional deflection, and add it to the rim sag found
previously, if both the face and rim indicators are to be used simultane-
ously. If the face and the rim indicators are to be used separately, to re-
duce sag, use the original rim sag in the normal manner, and use this
same original rim sag as shortly to be described in determining face sag
effect-in this latter case utilizing a rim indicator installed temporarily
with the face indicator for this purpose. If the face indicator is a different
type (Le., different weight) from the rim indicator, obtain rim sag using
this face indicator on the rim, and use this figure to determine face sag
effect.